The harmful effects of ultraviolet radiation upon human skin are well documented. Exposure to U.V.-B radiation, having a wavelength of 280 to 320 nm can cause sunburn or erythema. Excessive exposure to U.V.-A radiation, having wavelengths in the 320-400 nm range, can lead to rapid aging of the exposed skin or even pathological changes in the skin such as dermatosis.
In order to minimize such exposure, there have been developed cosmetic compositions incorporating compounds having the capacity to absorb ultraviolet radiation in the U.V.-A and U.V.-B ranges. However, many of the compounds currently commercially employed in such sunscreen cosmetic compositions have been found to cause irritation to sensitive skin. Moreover, while certain of these compounds will provide protection, it would nevertheless be desirable to possess sunscreen compositions comprising even more effective ultraviolet absorbing compounds.
While the polymer industry has developed many compounds which will absorb radiation in the critical range which will afford UV-stability to a wide range of materials, unfortunately very few of these compounds are suitable for use in human sunscreen compositions. Specifically, as is noted in U.S. Pat. No. 4,489,057 to Welters et al., at Col. 2, lines 34 et seq., several of the requirements for a U.V. stabilizer for a polymer composition are very different from those for a sunscreen agent for use in cosmetics. Thus, important criteria for cosmetic U.V. absorbers include good U.V. absorption, chemical stability, photostability, odorlessness, colorlessness, good tolerability by skin and mucous membranes, easy processability and good solubility in the solvents or bases commonly employed in the cosmetics industry. In contrast, very different criteria will be important for polymer-protective U.V.-stabilizers, for example, the ability to stop chain reactions in the polymers, with many of the requirements discussed above for cosmetic U.V.-absorbers being irrelevant for polymer use. Accordingly, obtaining U.V.-absorbing compounds suitable for sunscreen applications involves an art which is very different from that relating to polymer-stabilizing U.V. absorbers.
U.S. Pat. No. 4,371,651 to Leistner et al. discloses a broad class of 1,3-dicarbonyl compounds which are shown to enhance the resistance to deterioration by heat and light of polyvinyl halide resins. It has now been found that a narrow class of compounds within the broad disclosure of this patent exhibits desirable sunscreen activity--a finding which is unexpected as none of the members of this class of compounds are included amongst the large numbers of compounds which are identified in such publication as being particularly preferred for the stabilization of polymers against degradation.